This invention relates to a general method for the coupling of ring systems with easily oxidizable heteroatoms. More particularly, it relates to a novel method for synthesizing seleniumfulvalene derivatives by deselenization with alkoxy phosphorous bases of selenocarbonyls. It also relates to the preparation of the tetracyano-p-quinodimethane charge transfer salts of tetraselenofulvalene (TSeF) and diselenodithiofulvalene (DSeDTF) and their derivatives.
Considerable interest has been shown recently in the study of highly conducting organic charge transfer salts. The most attractive of these systems is the salt between tetrathiofulvalene (TTF) and tetracyano-p-quinodimethane (TCNQ) which displays exceptional electrical conductivity and metallic behavior over a wide temperature range. Further, a few anomalous crystals of TTF-TCNQ have been reported by L. B. Coleman et al, Solid State Commun. 12, 1125 (1973) to exhibit "giant" conductivity maxima. Attempts have been made to enhance the metallic properties of the tetrathiofulvalenium radical cation by substitution of electron-releasing substituents. Such substitution also results in considerable distortion of the TTF-TCNQ crystal structure. In order to improve upon the metallic properties of fulvalenium systems in a definable and controllable way, there has been devised a synthesis of tetraselenofulvalene (TSeF) and its charge transfer salt with TCNQ. The larger, more polarizable selenium reduces coulombic repulsion and increases electronic interaction, possibly by enhanced overlap of the cation radicals. Furthermore, this modification has essentially kept constant the original TTF-TCNQ crystal structure.
It has been found that the hitherto unknown tetraselenofulvalene (TSeF) cannot be synthesized in the same manner as its analogue, tetrathiofulvalene (TTF). The basic reactions used for TTF are shown in the publications to F. Wudl et al., Chem. Commun. 1453 (1970) and D. L. Coffen, et al., J. Amer. Chem. Soc., 93, 2258 (1971). The synthesis involves the oxidation of vinylene trithiocarbonate (1) followed by base coupling. The reaction sequence is outlined below in equation 1: ##STR2## The oxidation step in the above reaction sequence fails when easily oxidizable heteroatoms such as selenium are used in place of sulfur in compound 1.
Accordingly, it is an important aspect of this invention to provide a method for coupling ring systems having easily oxidizable heteroatoms in the ring system. It is a further object to provide a method for producing tetraselenofulvalene and related systems. It is yet another object to provide charge transfer salts prepared from TSeF and related systems with TCNQ.